This invention relates generally to wireless communication systems and more particularly to a method and system for downlink signal processing for an antenna array.
An adaptive signal processing system can be used to improve uplink performance, downlink performance, or both of an antenna array. Downlink performance refers to signal quality of a transmission from a base station to a subscriber station, whereas uplink performance refers to signal quality of a transmission from a subscriber station to a base station. An adaptive algorithm evaluates reception characteristics of one or more uplink signals to provide guidance in altering signal processing associated with antenna performance. For example, the uplink beam can be filtered or steered toward a direction that provides the best signal-to-noise ratio of the received uplink signals.
In the Background Art, adaptive signal processing techniques have more effectively improved uplink performance than downlink performance because uplink antennas and downlink antennas are frequently not symmetrical in design. For example, downlink antennas are often configured as three-sector configurations without spatial diversity, whereas uplink antennas can have more flexible radiation patterns and can feature spatial diversity. The uplink signal can fade differently than the downlink signal because of a frequency difference between the uplink signal and the downlink signal. Further, the time lag between uplink signal processing and downlink signal processing can lead to inaccuracies in the downlink signal processing which degrades the theoretically attainable signal-to-interference ratio for the entire wireless communication system.
Without commensurate increases or improvements in the uplink signal-to-interference ratio and the downlink signal-to-interference ratio, the frequency reuse of a wireless system does not realize the full potential of an adaptive array antenna. Thus, a need exists for an adaptive signal processing technique to facilitate increased frequency reuse and improved traffic capacity of wireless communication systems in both uplink and downlink modes of operation.
The invention, in part, provides an adaptive processing system (and method embodied therein) that can reduce interference perceived by the subscriber stations by controlling downlink radiation patterns of the base station with respect to out-of-cell subscriber stations. The downlink radiation patterns are controlled based on the interference and the desired signal received at the base station. No communications between base stations within the wireless communications system are necessary to reduce the system-wide interference. Thus, the adaptive processing system of the invention is well-suited for reducing system-wide interference without increasing inter-site overhead traffic to manage the reduction of interference.
In such a method of signal processing for an array antenna, uplink signals from the plurality of receiving/transmitting elements defining an array antenna are measured to determine covariance measurements between pairs of the uplink signals. A first covariance matrix is determined for a desired signal component of the measured uplink signals. A second covariance matrix is determined for a noise and interference component of the measured uplink signals.
Also related to such a method, an antenna processing system computes adaptive weights based on the first covariance matrix, the second covariance matrix, and a loading term to, in effect, form a desired directional downlink radiation pattern. The loading term represents a balance adjustment between a first performance rating of a particular coverage area versus a second performance rating of the wireless communication system, e.g., defined as a network of base stations serving a locality. The first performance rating can represent the extent that the processing system provides a desired downlink power of a downlink signal to a subscriber station within a particular coverage area, such as a sector or cell. The second performance rating can represent the extent that processing systems deployed throughout the wireless communication system suppress interference power to the downlink signal as perceived by the subscriber station in the particular coverage area. The interference power can result from neighboring coverage areas of the wireless communication system, for example.
Advantages of the present invention will become more apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.